It has been recognized in the luminaire art that it is advantageous, in certain types of luminaires, to provide dual light sources, the second of which may be tilized as a backup light source in the event of failure of the first or primary light source.
Desirable features of such an arrangement may include: (1) substantially identical operating and optical characteristics of the system, regardless of which light source is energized; (2) both light sources focusable, i.e. the dimensional relationship between the filament of the energized light source and the focal point of the optical system (reflector, lens, etc.) being adjustable along the focal, i.e., central axis of the optical system; (3) switching means to selectively energize and position the light sources, as well as to provide an "off" (neither source energized) position; and (4) switching, positioning and focusing preferably being accomplished by rotation of a single switch knob mounted externally of the luminaire apparatus.
Use of multiple light sources and means for energizing such light sources is well known in the art and has been disclosed, for example, in U.S. Pat. Nos. 1,184,400, 1,845,399, 2,079,732, 3,529,146, 3,529,147, 2,123,435 and 1,757,887.
More recently, there has been disclosed in U.S. patent application Ser. No. 886,783, which application has been assigned to the assignee of the present invention, a rotating socket arrangement carrying a plurality of bulbs combined with a housing body and operated by manual rotation of a knob member to selectively move one bulb out of a focal point of an optical system and another bulb into the said focal point while energizing same.
There is not disclosed in this earlier filed application means for providing individual focusing of either of the light sources. Similarly, the multiple light source arrangements of the prior art patents noted above do not provide for individual focusing of alternative light sources. To date no proposed system, so far as applicants are aware, has met all of the above criteria in a satisfactory manner.